The present invention is directed to bicycles and, more particularly, to various inventive features of a circuit used with bicycle dynamos.
Bicycles are equipped with dynamos for the purpose of illuminating headlamps, powering electrical components, and so on. The voltage generated by a dynamo is typically proportional to the speed of the bicycle which, in turn, is determined by the rate of rotation of the wheels. At high speed, the voltage can exceed 100 V in some instances. It is therefore necessary to design electrical components powered by the voltage generated by the dynamo to be able to withstand such high voltage. Unfortunately, components designed to withstand high voltages lack general application and tend to be expensive as well.
Another difficulty arising from the generation of voltages from a dynamo is fluctuation in the electrical load connected to a dynamo which, in turn, can result in bursts of extremely high voltage, termed “surge voltage”. Thus, a circuit for clamping voltage is needed to enable the use of standard electrical components and the like, as well as to protect components from extremely high voltage.
Conventional clamping circuits proposed to date include a circuit like that illustrated in FIG. 1. This conventional circuit is a bidirectional voltage clamping circuit having two Zener diodes. When positive voltage is output at the positive (+) terminal of dynamo GE, then element DZ1 functions as a Zener diode, and element DZ2 functions as a normal rectifier diode. Similarly, when positive voltage is output at the negative (−) terminal of dynamo GE, then element DZ2 functions as a Zener diode, and element DZ1 functions as a normal rectifier diode.
With the circuit illustrated in FIG. 1,Vc1=Vz1+Vf2  (1)where Vc1 is the clamping voltage for the dynamo voltage, Vz1 is the Zener voltage of element Dz1, and Vf2 is the forward voltage of element Dz2. When positive voltage is output at the negative (−) terminal of dynamo GE, the clamping voltage Vc2 for dynamo voltage is given byVc2=Vz2+Vf1.
Such a conventional clamping circuit has the advantage of relatively few parts. However, diodes Dz1 and Dz2 tend to generate heat, which can lead to problems in degraded characteristics. Assume, for example, that clamping voltage is set to Vc1=Vc2=10 V; semiconductor junction temperature Tj prior to clamping is 25° C.; current flow to elements Dz1 and Dz2 during clamping is constant; and semiconductor junction temperature at thermal equilibrium after commencing clamping is 100° C. When Vz1=9.1 and Vf2=0.9, at the instant of clamping, Equation (1) gives:Vc1=9.1+0.9=10 (V)whereas at thermal equilibrium, where the temperature coefficient αT=5 (mV/° C.),Vc1=9.1+(αT/1000)×(100−25)+0.9=10.375 (V).Thus, there is a significant variation in the clamped voltage.
To reduce degradation in characteristics due to heat generation, it is possible to design a circuit like that illustrated in FIG. 2. In this circuit, current flow is sensed by a current sensor element A. Loss P occurring in elements DZ1 and DZ2 is calculated from the current value Iz and clamping voltage Vc as follows:P=Vc×Iz. If loss P increases, then a switching element SW is opened to limit current flowing to the circuit.
However, such a circuit requires elements with high withstand voltage for the switch SW and sensor A controlling it. Another drawback is the increased number of components required. Furthermore, where a device is charged by the dynamo, failure to charge adequately may result from the operation of the switch.
Another possibility is a circuit like that shown in FIG. 3. In this circuit, the voltage Vty across the anode and cathode of a thyristor Th is sensed, and a trigger pulse is applied to the gate at the instant the voltage Vty exceeds a preset voltage. Application of a trigger pulse produces shorting (conduction) across the anode and cathode of the thyristor so that voltage Vty drops to around 0 V. Shorting continues until the current across the anode and cathode falls below the characteristic holding current of the thyristor.
However, this circuit design has problems, particularly when the signal produced by the dynamo is used to generate pulses that indicate bicycle speed. For example, FIG. 4(a) shows the waveforms generated during operation of a typical dynamo and speed sensing circuit; FIG. 4(b) shows the waveforms generated during operation of a dynamo and speed sensing circuit constructed in accordance with FIGS. 1 and 2; and FIG. 4(c) shows the waveforms generated during operation of a dynamo and speed sensing circuit constructed in accordance with the clamping circuit shown in FIG. 3. In each figure, Vs is the decision voltage for producing a speed sensing pulse, and the resultant pulse will have the shape shown at the bottom of each figure. The pulses produced by a dynamo and speed sensing circuit constructed in accordance with FIGS. 1 and 2 are the same as the pulses generated during the operation of a typical dynamo and speed sensing circuit. Thus, speed can be sensed with no particular problems using these pulses. However, when a clamping circuit has been designed with a thyristor-shorted circuit as shown in FIG. 4, clamping will produce a waveform like that shown at the top of FIG. 4(c) due to the tendency to drop to 0 V when clamped by the clamping thyristor. As a result, the pulse produced by decision voltage Vs will have a disturbed waveform like that shown at the bottom FIG. 4(c), thus making it impossible to sense speed accurately.